Most efforts to address groundwater contamination in subsurface media have historically involved pumping to extract contaminated groundwater from the subsurface for above-ground treatment. This process is commonly referred to as "pump-and-treat". However, pump-and-treat systems are limited in their ability to remediate contaminated groundwater, soil, and rock (Mackay et al., Environmental Science and Technology, 23(6) : 630-636 (1989) and Travis et al., Environmental Science and Technology, 24: 1464-1466 (1990)). Remedial lifetimes are commonly on the order of decades to centuries. During pumping, there are ongoing energy and cost requirements, and the need to properly treat the groundwater for disposal at the surface poses an additional problem.
Conventional pump-and-treat systems are particularly ineffective for collecting and treating groundwater from low-permeability fractured rock. Contaminated groundwater in low-permeability fractured rock is commonly extracted for treatment at the surface by actively pumping from one or more conventional vertical recovery wells, typically installed at or near the contaminant source for source remediation and at the downgradient end of a contaminant plume to control contaminant migration.
Conventional migration control in low-permeability fractured rock typically requires installation of a sufficient number of recovery wells to capture all of the contaminated groundwater that would otherwise pass by. However, when the fractures in the low-permeability rock are irregularly spaced and poorly interconnected, as they commonly are, large numbers of conventional vertical recovery wells must be placed fairly close together to ensure that most of the contaminated groundwater is captured. Sometimes, the range of potential groundwater capture in low-permeability fractured rock for a single well may be only ten to thirty feet, especially under unconfined conditions, and attempting to achieve effective capture over a wide region may require several dozen wells equipped with pumps and piping, all of which must be installed and maintained at considerable expense. Moreover, in cold climates, the associated surface lines and other equipment must be protected from freezing which further adds to the expense. Even with close well installation, some large fractures or faults may pass unnoticed between wells and allow significant migration of contaminants past the area of pumping.
In recent years, unconventional horizontal wells, rather than conventional vertical wells, have sometimes been used for groundwater recovery, but, in general, these also require active pumping to extract contaminated groundwater, and they are very expensive to construct. Moreover, horizontal wells frequently fail to capture contaminated groundwater in low-permeability fractured rock, because the presence of certain extremely low-permeability horizontal rock layers prevents groundwater from flowing at an appreciable rate in the vertical direction, except through widely-spaced isolated fractures. Under the hydraulic stress of pumping, there might be negligible or very small vertical components of flow through these low-permeability layers to the horizontal-well screens, precluding effective groundwater recovery from horizontal rock layers that are separated from the well screen by the low-permeability layers.
A comparatively new method for recovering groundwater from low-permeability fractured rock involves pumping groundwater from a linear blasted-bedrock zone or from a radial set of blasted-bedrock zones (Begor et al., Ground Water, 27: 57-65 (1990); Smith et al., Proceedings of the 5th Annual Hazardous Materials and Environmental Management Conference/Central, Chicago, pp. 103-117 (1992); Gehl, Proceedings of the Focus Conference of Eastern Regional Groundwater Issues, National Water Well Association, pp. 265-273 (1994); and McKown et al., Proceedings of the 21st Annual Conference on Explosives and Blasting Techniques, International Society of Explosives Engineers, pp. 305-322 (1995) ("McKown")) The linear zones of shattered rock, commonly referred to as "trenches", along with one or more pumping wells, can form an extraction system that effectively connects otherwise naturally unconnected fractures and greatly increases the effective region of pumping influence. Since a blasted-bedrock trench has a much greater hydraulic conductivity than the native rock surrounding it, groundwater flows in from many directions when the pump in a trench recovery well is operating. The average flow rate from one or more recovery wells installed in a blasted-bedrock trench exceeds the hypothetical flow rate of more than 60 or 70 traditional recovery wells installed in the same type of rock (McKown). However, as indicated above, flow into the trench is effected only when the trench is actively pumped.
One common characteristic of the groundwater "pump-and-treat" methods, irrespective of whether the well employed is a horizontal well, a vertical well, or a blasted-bedrock well, is the need to bring the groundwater collected by the well to the surface for treatment. Generally, this requires more or less continuous pump operation, and this pumping process, as well as the surface treatment processes, is expensive in terms of operational and maintenance costs. In addition, effluent from the above-ground treatment of groundwater typically must be discharged to a permitted discharge point or to a publicly owned wastewater treatment facility. Since the treated groundwater is discharged above ground, chemical substances other than the contaminants which necessitated the groundwater treatment must also be dealt with in the treatment process and be remediated to levels acceptable for above-ground discharge. For example, when Fe(II) in anaerobic groundwater is brought to the surface and exposed to air, it commonly is oxidized to Fe(III) which then forms a precipitate, which can cause water-quality problems and foul treatment equipment and lines. Another example of this problem is the need to permit colloidal materials and silt to settle out the groundwater prior to treatment and/or discharge. Thus, above-ground treatment frequently necessitates the installation of systems to treat substances which would never have needed remediation had the water remained in the subsurface. The long-term costs of above-ground effluent treatment and disposal, coupled with the long-term costs associated with the maintenance and operation of pumps and related equipment, makes above-ground treatment of groundwater expensive.
Therefore, a continuing need exists for methods of directing groundwater flow and treating groundwater. The present invention is directed to meeting this need.